Conventional casting methods such as die casting, gravity permanent mold casting, and squeeze casting have long been used for Aluminum-Silicon (Al—Si) alloys. However, where semi-solid metal (SSM) casting of Al—Si alloy materials has been involved, the conventional methods have not been employed successfully to date. Rheocasting and thixocasting are casting methods that were developed in an attempt to convert conventional casting means to SSM casting. However, these SSM methods require costly retrofitting to conventional casting machinery.
Challenges also remain in the ability to manipulate the mechanical and metallurgical properties of SSM castings. As the performance of the cast product is predicated, in part, by the microstructures of primary Al and/or Si particles in the part, attempts have been made to improve methods to achieve the requisite microstructure. One approach is to achieve homogeneous distribution of primary Al or Si, while another is to limit the growth and size of the particles themselves.
The physical characteristics of the primary particles depends on the imposed temperature gradient, presence of impurities, and ease of nucleation. Known strategies to affect these parameters include the use of electromagnetic stirring and grain refiners, such as titanium alloys. Alternatively, control of the cooling rate and isothermal hold time of the alloy at the SSM temperature can also affect the microstructures. Most, if not all, of the research in this regard has been, however, related to conventional casting of Al—Si alloys and little has been employed in SSM casting of Al—Si alloys.
Accordingly, it is desirable to provide a method of casting SSM Al—Si alloys utilizing both conventional and rheocasting means that can impart desirable mechanical properties. In particular, there is a need for controlling the nucleation of primary Al particles in Al—Si alloys to limit the formation of large primary Al particles.